Fewer than half of adult women in the US engage in recommended levels of physical activity. As a consequence, they are at increased risk of heart disease, stroke, hypertension, diabetes, certain cancers, and premature mortality. Given the rapid growth of mobile phone technology and the increasing number of users, we developed an interactive mobile phone-based physical activity intervention for sedentary women (based on the Choose to Move program developed by the American Heart Association for sedentary women). We assessed the feasibility and acceptability of this intervention in a 3-week pre-post study in which we had rapid enrollment, no attrition, high compliance (95% compliance with a pedometer and 85% compliance with a cell phone diary), and a significantly increased step count. We now propose to conduct a randomized, controlled trial to assess the efficacy of the mobile phone-based physical activity intervention on increasing physical activity over a 3-month period. 192 sedentary women will be randomized in a 2-to-1 ratio to a 3-month mobile phone-based physical activity intervention group or to a control group. Participants in both groups will receive a pedometer and a mobile phone and will asked to wear a pedometer and to send their total number of steps through a cell phone diary before going to bed each night. Only participants in the intervention group will receive the Choose to Move program through their cell phones via daily prompts, weekly video clips, and customized feedback and self-monitoring. The primary outcome will be total steps per day (as measured by the pedometer). Secondary outcomes will be the aerobic steps per day (also measured by the pedometer) and self-reported 7-day physical activity recall. To provide insight into how best to maximize the potential for sustained physical activity after completion of the 3-month program, women in the intervention group who complete the physical activity program will be further randomized into a 6-month maintenance intervention-PLUS program (pedometer plus cell phone diary) or to a 6-month maintenance intervention-REGULAR program (pedometer only). Unlike a conventional maintenance follow-up in which all subjects continue in their respective randomization arms, re-randomizing subjects into either a maintenance intervention-REGULAR group or a maintenance intervention-PLUS group will allow us to examine the dose-response of the maintenance methods if the intervention is effective. We will also explore the role of potential mediating factors (physical activity, self-efficacy, social support, outcome expectation, and decisional balance) and moderating factors (BMI and age) on changes in physical activity. If proven effective, a major advantage of a mobile phone-based intervention is that it could potentially be administered to a large number of women.